It is known to attach a device to a computer mouse in order to be able to use the mouse for tracing images or digitizing. These devices work by maintaining the mouse fixed at a given angle with respect to the plane on which the mouse is moved. U.S. Pat. No. 4,561,163 describes how this is done and shows the use of drafting table arms to accomplish the fixing of the mouse angle. U.S. Pat. No. 4,831,736 describes a mouse carriage having wheels whose steering angles are maintained to be the same such that the angle with respect to the plane on which the carriage is moved remains fixed. In the two above mentioned U.S. patents, the cost of the devices is substantial, since their construction is rather complicated.